I have a basic understanding how you can create checklists. 

The problem is, that when I try to add any kind of checklist the ingame internative checklist panel stays blank. I put the XML file here: F:\Microsoft Flight Simulator\Packages\Community\fnx-aircraft-320\SimObjects\Airplanes\FNX320\Checklist

This is the place where MSFS is installed. Checklists for default planes work normal, so I don't think it is something setting related. I used the most basic example from here: https://docs.flightsimulator.com/html/Content_Configuration/Checklists/Checklist_Examples.htm

Which is: 

<?xml version="1.0" encoding="Windows-1252"?> <SimBase.Document Type="Checklist" version="1,0"> <Checkpoint> <CheckpointDesc SubjectTT="TT:GAME.CHECKLIST_PARKING_BRAKE" ExpectationTT="TT:GAME.CHECKLIST_SET"/> </Checkpoint> </SimBase.Document>

Can anyone with the Fenix try to create a working example like that? There is nearly no information on Google about that issue, so troubleshooting is very hard. 

I tried this as well and failed.  I have made a number of checklists for other planes without a problem...but for some reason this file is not being read when the plane loads.

In the meantime there is a complete MSFS Ingame Checklist for the FENIX A320 available from https://flightsim.to/file/74681/fenix-a320-ingane-checklist .Please take a look.